1. Technical Field
The technical field relates to a low frequency cutoff filter which is used for a camera shake correction for an imaging apparatus such as a digital still camera or a digital video camera.
2. Related Art
Some of imaging apparatuses such as digital still cameras and digital video cameras have a camera shake correction function. Such imaging apparatuses have a camera shake correction system (stabilizer) which calculates a camera shake angle by integral of an angular rate detected by an angular rate sensor and drive a correction lens or an imaging device according to the camera shake angle, thereby canceling a camera shake (see, for example, JP2007-324929A). A detection output from the angular rate sensor includes extremely low frequency fluctuating components in addition to components of a camera shake angular rate desired to be detected. Hence, in the camera shake correction system, a low frequency cutoff filter for cutting off a direct current component is provided at a stage preceding an integral process.
In addition, in the camera shake correction system, an integrator is provided to convert an angular rate detected by the angular rate sensor to an angle. It is desirable that the integrator ideally have complete integral characteristics in a camera shake correction band (about 1 to 20 Hz) which is a frequency range where the camera shake correction function is performed. However, when the integrator has complete integral characteristics even for extremely low frequencies, an extremely low frequency fluctuating component from the angular rate sensor is also amplified and thus excellent camera shake correction performance cannot be obtained. Therefore, the integrator has, at the extremely low frequencies, the characteristics of a first-order lag system in which the band is limited to prevent gain from increasing, and has a phase lead at 1 Hz as a lower limit of the camera shake correction band.
Meanwhile, conventionally, various filter technologies for performing phase compensation have been developed (see, for example, JP2006-340190A). JP2006-340190A discloses means to provide the best phase characteristic in a passing band with a basic filter unit and a second or higher order filter.
As described above, in conventional imaging apparatuses, in an output signal from the angular rate sensor, the integral band of the integrator is limited and furthermore a low frequency cutoff filter is provided, in order to reduce extremely low frequency fluctuating components which are included in the output of the angular rate sensor and are other than an angular rate component. Such limitation on the integral band of the integrator and the low frequency cutoff filter cause the phase on the low frequency side of the camera shake correction band to advance. Therefore, the phases of the actual camera shake angle and a correction angle do not match each other, resulting in a large correction residual. Accordingly, there is a problem that a sufficient suppression effect cannot be secured.
Although there is another method in which the order of the filter is increased to allow the phases to match each other, it has problems, for example, that the increase in the order of the filter complicates a computation algorithm and that a circuit becomes complicated when the filter is configured by hardware.